Formed fiber containers, such as paper plates and trays, are commonly produced either by molding fibers from a pulp slurry into the desired form of the container or by pressing a paperboard blank between forming dies into the desired shape. The molded pulp articles, after drying, are fairly strong and rigid but generally have rough surface characteristics and are not usually coated so that they are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers, on the other hand, can be decorated and coated with a liquid-proof coating before being stamped by the forming dies into the desired shape. Pressed paperboard containers generally cost less and require less storage space than the molded pulp articles. Large numbers of paper plates and similar products are produced by each of these methods every year at relatively low unit cost. These products come in many different shapes, rectangular or polygonal as well as round, and in multicompartment configurations.
Pressed paperboard containers tend to have somewhat less strength and rigidity than do comparable containers made by the pulp molding processes. Much of the strength and resistance to bending of a plate-like container made by either process lies in the sidewall and rim areas which surround the center or bottom portion of the container. When in use, such containers are supported by the rim and sidewall while the weight held by the container is located on the bottom portion. Thus, the rim and sidewall generally are placed in tension and flexure when the container is being used.
In plate-like structures made by the pulp molding process, the sidewall and overturned rim of the plate are unitary, cohesive fibrous structures which have good resistance to bending as long as they are not damaged or split. Because the rim and sidewall of the pulp molded containers are of a cohesive, unitary structure, they may be placed under considerable tension and flexure without failing.
In contrast, when a container is made by pressing a paperboard blank, the flat blank must be distorted and changed in area in order to form the blank into the desired three dimensional shape. This necessary distortion results in seams or pleats in the sidewall and rim, the areas of the container which are reduced in press forming the container. These seams or pleats constitute material fault lines in the sidewall and rim areas about which such containers bend more readily than do containers having unflawed sidewalls and rims. Moreover, such seams or pleats have a tendency to return to their original shape--flat. The necessary location of these pleats in the sidewall and rim of pressed paperboard containers places the greatest weakness in the area requiring the greatest strength. Such containers have been unable to support loads comparable to pulp molded containers since, when in use, the greater the load is, the higher the stress imposed on the rim and sidewall. Imposing tension, flexure or torsion on pleats merely enhances their tendency to open. Accordingly, known pressed paperboard containers have significantly less load carrying ability than do pulp molded containers. Being less costly than an equivalent pulp molded plate, a pressed paperboard plate with comparable strength and rigidity would have significant commercial value.
Many efforts have been made to strengthen pressed paperboard containers while accommodating the necessary reduction in area at the sidewalls and rims. Blanks from which paperboard containers are pressed have been provided with score lines at their periphery to eliminate the random creation of seams or pleats. The score lines define the locations of the seams or pleats. Score lines, sometimes in conjunction with special die shapes, have been used to create flutes or corrugations in the sidewall and rim for aesthetic and structural purposes. The additional cost and complexity of dies used to create flutes or corrugations in the sidewall of such containers presents a cost disadvantage which may not be entirely justified.
Whether the area reduction of the sidewall and rim is accommodated by pleats, seams, flutes or corrugations, the basic difficulty has been that under limited stress the paperboard will tend to return to its original shape.
To overcome this tendency, it has been suggested that the rim be subjected to various strengthening techniques. The earliest efforts comprised the addition of several thicknesses of paperboard at the rim. This container, however, required additional manufacturing steps and increased the cost and required storage space of the containers.
More recently, as disclosed in commonly assigned U.S. Pat. No. 4,609,140 issued to Van Handel et al., improved rigidity in a pressed paperboard container has been achieved by application of pressure and temperature to the rim of the container while applying substantially no pressure to the sidewall and bottom wall. In particular, the container had a generally planar bottom wall, a sidewall upwardly rising from the bottom wall periphery and an overturned rim extending from the sidewall periphery. During integrally press-forming of the container, substantially no pressure was applied to the bottom and sidewalls and pressure was applied to the overturned rim. The amount of pressure imposed on the rim was approximately 200-250 psi and gradually increased from the juncture of the rim and sidewall to the peripheral edge of the rim. The pleats formed in the rim were compressed to the thickness of the rim while the pleats formed in the sidewall were not subjected to any significant pressure. The container thus formed provided a significant improvement over prior paperboard containers. Additionally, commonly assigned U.S. Pat. No. 4,606,496 issued to Marx et al. discloses an improvement over that set forth hereinabove. Therein, a container is disclosed which includes densified regions radially extending through and circumferentially spaced about annular sections of the rim. The densified regions are formed from pleats including at least three layers of paperboard which are created during press forming of the blank into a container, the rim region being subjected to sufficient pressure to form the pleats into cohesive fibrous structures having a density substantially greater than the density of the area of the rim adjacent the pleats while still being of substantially the same thickness.
While the paperboard container formed in accordance with that process set forth in U.S. Pat. No. 4,606,496 exhibits a greater rigidity than that of the remaining prior art, the containers do not have the strength of pulp molded containers, thus, there remains a need for yet a stronger, more stable pressed paperboard container which will reliably resist bending when food is placed thereon by the consumer. Further, there is clearly a need for a stronger, more rigid paperboard container which resists bending and which can be manufactured in a cost effective manner.